The present invention relates to a microwave cavity and, more specifically, to a microwave cavity having a power coupling probe for optimizing the impedance match between the cavity and the source of microwave power.
Various types of adjustable microwave cavities are known in the prior art. Examples of these may be found in the publication by Fehsenfeld et al entitled "Microwave Discharge Cavities Operating at 2450 MH.sub.z ", Review of Scientific Instruments, Vol. 36, No. 3, (March 1966). One cavity described therein is shown in FIG. 1 of the drawings herein and is known as an "Evanson Cavity" which is manufactured by the Opthos Manufacturing Company of Rockland, MD. In this cavity 10, the power coupling stub consists of a fixed brass probe 12 which extends from an input connector 14 to the wall 16 of the cavity. A brass tube 18 which is threaded at the outer end 20 is then attached to the probe 12 so as to make a sliding electrical contact with the fixed probe. This threaded end is screwed into an insulator 22, such as teflon or ceramic material, which protrudes through a hole 24 in the main cavity. Adjustment of the coupling probe penetration in this type of cavity is made by grasping the protruding insulator 22 and pulling or pushing so as to move the effective electrical probe tip along the diameter of the main cavity 16.
There are several disadvantages in this cavity which makes its use for some applications less than optimal. From the point of view of safety, it is not desirable to handle a coupling probe extension. A considerable amount of microwave power (100's of watts) is routinely applied to the cavity through the coupling probe and leakage can occur through the hole 24 for the insulating extension 22. Furthermore, the insulator 22 can become quite hot when the cavity contains an electrodeless discharge, and can not be handled without gloves. Another problem with this type of adjustable cavity is that the effective probe tip position is not readily obvious since it is screwed into the insulator. Therefore, probe penetration must be obtained by some kind of indirect measurement. Also, the effective diameter of the probe is some value between the diameter of the fixed probe 12 and that of the brass tube 18 depending upon their relative positions. This, of course, affects the characteristic impedance of the input arm since this impedance is dependant upon the ratio of the inner diameter of the outer conductor to the outer diameter of the inner conductor in a coaxial line. An additional disadvantage of this type of cavity is that the hole 24 is necessary in the wall of the main cavity 16 to allow adjustment of the probe depth. This coupling technique causes the field configuration within the cavity to be more complex and difficult to control, because the hole introduces a discontinuance in the wall of the cavity, and the probe extension introduces an unnecessary dielectric inside the cavity and a possible source of power loss.